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SSRI-Induced SIADH — RACP Adult Medicine MCQ

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ModerateNeurologySSRI-Induced SIADHRACP Adult Medicine

A 60-year-old man on an SSRI (sertraline) for depression presents with confusion and Na⁺ 118 mmol/L. Serum osmolality 252 mOsm/kg. Urine osmolality 450 mOsm/kg (inappropriately concentrated). Urine Na⁺ 55 mmol/L. He is euvolaemic. What is the most likely cause?

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Correct answer: B — SSRI-induced SIADH

Euvolaemic hyponatraemia with low serum osmolality, inappropriately concentrated urine (>100 mOsm/kg), and high urine sodium (>30 mmol/L) on an SSRI is SIADH. SSRIs are a common drug cause of SIADH (all SSRIs, onset typically 2-4 weeks). Management: fluid restriction (1-1.5 L/day), cease the offending SSRI if possible, and switch to an alternative antidepressant (mirtazapine or bupropion have lower SIADH risk).

Reference: AMH – 2025 – SSRI-Induced Hyponatraemia; European Hyponatraemia Guidelines 2023